Understanding Construction Waterproofing: Positive, Negative and Blindside

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two workers laying out waterproofing materials

A waterproofing membrane is a thin layer of waterproof materials whose purpose is to prevent water from entering the surface its protecting. In construction, waterproofing membranes are laid on foundations, roofs, and walls since moisture could compromise the structural integrity of those structures. Before installing a waterproofing membrane, the contractor has to make considerations such as the choice of the waterproofing material and the location of the waterproofing membrane. Common types of waterproofing membranes are liquid, bituminous, and cementitious waterproofing membranes. These membranes can either be installed on the outer face, the interior face, or on the soil retention system. Below are the three types of waterproofing according to the location of the waterproofing membrane.

Positive Waterproofing

This is the most common type of waterproofing in modern construction; the waterproofing membrane is installed on the exterior face of the structure, which is also referred to as the wet face of a structure. Positive waterproofing membranes can be applied above or below grade. These membranes prevent water from the environment or the surrounding soil from infiltrating the structures, thus protecting structural components such as steel and concrete. Positive waterproofing membranes also protect structures from freeze-thaw cycles and corrosive chemicals that are sometimes present in groundwater.

The main disadvantage of positive waterproofing is that it’s difficult and expensive to repair after installation, especially when it’s below grade. This is because repairing it requires removal of the topping.

Negative Waterproofing

waterproofing foundation building

In negative waterproofing, the membrane is installed on the inner side of structures, also referred to as the dry face. The primary purpose of negative waterproofing is to hold water, thus preventing moisture from infiltrating into occupied space. Epoxy injections and cementitious waterproofing membranes are preferred in negative waterproofing because they can withstand high hydrostatic pressure.   The main advantage of this type of waterproofing is that the waterproof membranes are easily accessible for repairs and improvement after installation.

In construction, negative waterproofing membranes do not prevent water from infiltrating the substrate, and this is both an advantage and a disadvantage depending on how you look at it. It’s as an advantage because it allows continuous curing of concrete substrates and a drawback because it could weaken the structure due to infiltration of corrosive chemicals.

Blindside Waterproofing

In blindside waterproofing, the waterproofing membrane is usually laid on the soil retention system. This type of waterproofing is used when there are space limitations such that the membrane cannot be applied after pouring the concrete; for example, in places where buildings have already been erected on the neighboring plots or when constructing tunnel-like structures. The waterproofing material is first applied on the water retention system and then the concrete is poured. Blindside waterproofing minimizes destruction of the surrounding land; it’s, therefore, the preferred type of waterproofing in green construction.

Negative Vs. Positive Waterproofing

The choice of the type of waterproofing material depends on the nature of the construction site. Positive waterproofing is generally more effective in protecting structures than negative waterproofing. Blindside waterproofing can be considered a type of positive waterproofing since the waterproofing membrane is on the wet face of the structure. Positive waterproofing is applied when the structure is exposed to corrosive chemicals, interior humidity limitations, and freeze-thaw cycles; otherwise, negative waterproofing can be used.

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